The present invention is directed toward the field of temperature controllers. In particular, a dual-setpoint temperature controller is disclosed that controls the temperature of a device at two particular setpoints that are modulated by the output of a humidity detection sub-system.
The preferred embodiment of the present invention is particularly useful for controlling the heating element of a clothes dryer as a function of temperature and relative humidity ("RH"). This same circuitry could be used with other appliances or devices in which temperature and RH are used as control variables, and could also be used with devices in which temperature and some other measurable parameter are used as control variables.
In the preferred clothes dryer application, it is desirable to automatically control the temperature in the dryer so as to minimize heat damage to the clothes, and at the same time to minimize power consumption of the dryer. When the dryer is first activated (beginning of a cycle), the clothes are very wet, and therefore the RH in the dryer will be quite high (i.e. greater than 80%). But, as the clothes begin to dry, and moisture is removed from the dryer system, the RH measured in the dryer will drop. At the same time that the RH is dropping, the temperature in the dryer typically rises, potentially causing heat damage to the clothing. At some point near the end of the drying cycle, the RH drops to a point where it is desirable to turn off the heater element in order to prevent damage to the clothes, and also to conserve energy.
Presently known techniques for controlling the dryer heater as the RH drops to a low value include: (1) resistive strips on the interior of the dryer drum, which are sensed in order to determine whether the clothes are moist and if not, to shut off the heater element for a certain period of time; (2) a bi-metal thermostat positioned in the exhaust of the dryer that detects the rise in temperature of the exhaust, which is typically associated with a drop in RH in the drum, and which can be used to turn off the heater element for some period of time; and (3) microprocessor controlled systems that measure temperature and RH and perform some complex calculations in order to determine whether or not to turn the heater element off as the RH drops below a particular threshold value. Each of these techniques suffers from several disadvantages that are overcome by the present invention.
The resistive strip technique typically requires some type of sophisticated signal conditioning and monitoring circuitry. Since the moist clothes only momentarily come into contact with the resistive strips, circuitry must be included that senses these momentary circuit connections, and which counts the number of connections per unit time in order to infer the RH of the clothes. A microprocessor or microcontroller is typically utilized for this function. This is a costly device. The thermostat technique only approximates the actual RH in the exhaust, and may, under certain conditions, be an inaccurate method to infer clothes wetness. And as noted with the resistive strip technique, other microprocessor-based solutions that actively measure both temperature and RH are also more complex and costly solutions. In addition to these disadvantages, none of the presently known techniques enables a gradual reduction in the exhaust temperature of the dryer after a particular RH threshold has been reached, as does the present invention.
Therefore, there remains a general need in this art for a cost-effective sensor modulated temperature controller, in particular one in which the sensor is a RH detector.
There remains a more particular need for such a temperature controller that nominally controls temperature at two initial setpoints, and which is responsive to a change in measured RH beyond a particular threshold value by modifying the setpoints.
There remains yet an additional need for such a temperature controller for use with the heating element of a clothes dryer or other appliance in which the measured RH causes a reduction in the dual setpoints of the temperature controller beyond a particular threshold value in order to: (1) minimize heat damage to the clothes in the dryer; and (2) conserve energy.